Surface cutting method

ABSTRACT

The present invention relates to a surface cutting method for cutting a surface within an area (AR) bounded by a predetermined closed curve (CCL) by moving a tool (TL) along a cutting path (PT i ) in a predetermined direction (direction of arrow A) to cut the surface within the area, thenceforth moving the tool along an adjacent cutting path (PT i+1 ), obtained by a shift of a predetermined amount, to cut the surface, and repeating these surface cutting operations. 
     The surface cutting method includes obtaining an offset curve (OFC) offset by a predetermined amount to the outer side of the closed curve (CCL), obtaining width W, in the shift direction (direction of arrow B), of the area bounded by the offset curve (OFC), finding, from among lengths of line segments obtained by dividing the width W into n equal parts, a length closest to a predetermined maximum cut-in amount without exceeding the same, adopting this length as an actual cut-in amount P, and performing surface cutting by shifting the tool (TL) by the cut-in amount P in the shift direction after the end of surface cutting along the cutting path PT i , and thereafter moving the tool along the adjacent cutting path PT i+1 . A cutting starting point (P i ) and cutting end point (Q i ) of each cutting path are provided on an offset curve (OFC&#39;) offset by a predetermined amount to the outer side of the closed curve (CCL) specifying the area.

TECHNICAL FIELD

This invention relates to a surface cutting method and, moreparticularly, to a surface cutting method for cutting a surface withinan area bounded by a closed curve by moving a tool along a cutting pathto cut the surface within the area, thenceforth moving the tool along anadjacent cutting path, obtained by a shift of a predetermined amount, tocut the surface, and repeating these surface cutting operations.

BACKGROUND ART

A form of numerically controlled machining is one in which there is anarea bounded by a closed curve, wherein surface cutting is applied to aportion (convex portion) projecting from an area on the outer side ofthe closed curve.

Such an area cutting (surface cutting) method includes the followingsteps:

(a) inputting data specifying a closed curve CCL of an area AR shown inFIG. 6, cutting direction (direction of arrow A) along cutting pathPT_(i) (i=1, 2, . . . ), shift direction (direction of arrow B) in whicha tool TL is shifted by a predetermined amount of cut-in wheneversurface cutting along the cutting path PT_(i) ends, and cut-in amount P;

(b) generating the cutting path PT_(i) on the basis of the inputteddata;

(c) performing cutting by moving the tool along the cutting path PT_(i)from a cutting starting point P_(i) to a cutting end point Q_(i) on thegenerated cutting path PT_(i) ;

(d) obtaining the next cutting path PT_(i+1), which results when thetool is shifted by the amount of cut-in P following the end of cuttingalong the preceding cutting path;

(e) thereafter performing cutting (unidirectional cutting) by moving thetool from point P_(i+1) to point Q_(i+1), in which point P_(i+1) istaken as the cutting starting point of cutting path PT_(i+1) and pointQ_(i+1) is taken as the cutting end point of cutting path PT_(i+1), orperforming cutting (back-and-forth cutting) by moving the tool frompoint Q_(i+1) to point P_(i+1), in which point Q_(i+1) is taken as thecutting starting point of cutting path PT_(i+1) and point P_(i+1) istaken as the cutting end point of cutting path PT_(i) +1 ; and

(f) repeating the unidirectional or back-and-forth cutting operationfrom this point onward to surface-cut the area AR.

In the conventional surface cutting method, the positions of the cuttingstarting point and cutting end point of each cutting path PTi are setappropriately. This results in a long tool pass, namely in a lengthyperiod of time during which surface cutting is not carried out.Consequently, machining efficiency is poor.

In addition, since the actual cut-in amount P is decided appropriatelyin the prior art, the amount of cut-in is too small and results in alarge number of cutting strokes (cutting paths) and diminished cuttingefficiency, or the amount of cut-in may be non-uniform (e.g. the lastcut-in being very small in comparison with the others).

Accordingly, an object of the present invention is to provide a surfacecutting method through which tool pass can be shortened, the number ofcutting strokes reduced and the amount of cut-in made uniform.

DISCLOSURE OF THE INVENTION

The present invention relates to a surface cutting method for cutting asurface within an area bounded by a predetermined closed curve by movinga tool along a cutting path in a predetermined direction to cut thesurface within the area, thenceforth moving the tool along an adjacentcutting path, obtained by a shift of a predetermined amount, to cut thesurface, and repeating these surface cutting operations.

The surface cutting method includes obtaining an offset curve offset bya predetermined amount to the outer side of the closed curve, obtainingwidth W, in a shift direction, of the area bounded by the offset curve,finding, from among lengths of line segments obtained by dividing thewidth W into n equal parts, a length closest to a predetermined maximumcut-in amount without exceeding the same, adopting this length as anactual cut-in amount P, and performing surface cutting by shifting thetool by the cut-in amount P in the shift direction after the end ofsurface cutting along a cutting path PT_(i), and thereafter moving thetool along an adjacent cutting path PT_(i+1).

The cutting starting point and cutting end point of each cutting pathare provided on an offset curve offset by (T+C+R) to the outer side ofthe closed curve specifying the area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for describing the general features of the presentinvention;

FIG. 2 is a block diagram of an apparatus for realizing the presentinvention;

FIG. 3 is a flowchart of processing indicative of the surface cuttingmethod of the present invention;

FIG. 4 is a view for describing offset processing;

FIG. 5 is an explanatory view illustratng a tool radius R and effectivetool radius R ; and

FIG. 6 is a view for describing the prior-art method.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a view for describing the general features of the presentinvention. In the Figure, CCL represents a closed curve, and AR (theshaded portion) denotes the area to undergo surface machining bounded bythe closed curve CCL. OFC designates an offset curve offset to the outerside of the closed curve CCL by the sum of excess thickness T and amountof clearance C. OFC' is an offset curve offset to the outer side of theclosed curve by (T+C+R), where R is the tool radius. Re designates theeffective tool radius. The arrow A indicates the direction of thecutting path, arrow B indicates the direction of a shift, W denotes thewidth of the area A in the shift direction, PT_(i) (i=1, 2 . . . )denotes a tool path, and P represents the amount of cut-in.

The area AR can be surface-cut reliably and tool pass in the cuttingpath direction can be minimized in length if the cutting starting pointP_(i) and cutting end point Q_(i) of each cutting path PT_(i) (i=1, 2, .. . ) are situated on the offset curve OFC' offset, to the outer side ofthe closed path CCL specifying the area AR to be surface-cut, by(T+C+R).

When the length of line segments, obtained by dividing theshift-direction width W of the area AR into n equal parts, is nearest apredetermined maximum cut-in amount without exceeding the same, surfacecutting can be performed at a uniform cut-in amount (W/n every stroke)if the aforementioned length is adopted as the actual cut-in amount P.This will also make it possible to perform surface cutting bysuccessively moving the tool a small number of times (=n times), i.e.along n cutting paths.

Therefore, in accordance with the invention, the first step is to obtainthe offset curve OFC offset by the predetermined amount (=T+C) to theouter side of the closed curve CCL.

Next, the width W in the shift direction (the direction of arrow B) ofthe area bounded by the offset curve OFC is found, and so is the lengthof line segments, which are obtained by dividing the width W into n (nis an integer) equal parts, closest to a predetermined maximum cut-inamount without exceeding the same. The length is adopted as the actualcut-in amount P.

Thereafter, the position Y of the i-th cutting path PT_(i) in the shiftdirection is obtained in accordance with the following equation:

    Y=Y.sub.0 +W-i·P+R.sub.e

where Y₀ represents the position of the lowermost end of the area AR inthe shift direction, and R_(e) represents effective tool radius.

When the position Y of the cutting path PT_(i) has been found, points ofintersection P_(i), Q_(i) are obtained between the cutting path PT_(i)and the offset curve OFC' offset by (T+C+R) to the outer side of theclosed curve CCL, where represents excess thickness, C amount ofclearance and R tool radius.

Surface cutting is performed along the cutting path PT_(i) from thepoint of intersection P_(i) to the point of intersection Q_(i). Surfacecutting along this path is executed n times.

FIG. 2 is a block diagram of an embodiment of the invention, and FIG. 3is a flowchart of processing according to the invention. The areacutting method of the invention will now be described with reference toFIGS. 1, 2 and 3.

(1) When a cycle start button on an operator's panel 101 is pressed, aprocessor 102 causes an NC data reader 103 to read one block of NC datafrom an NC tape 104. The NC tape 104 stores area cutting (surfacecutting) data in addition to ordinary path data, G-function instructiondata and M-, S- and T-function instruction data. Stored at the end ofthe NC program is an M code (M02) indicating program end. Placed at thebeginning of the area cutting data is an area cutting command indicatingthat the data which follow it are the area cutting data. Placed at theend of the area cutting data is a code indicative of the end of the areacutting data.

(2) The processor 102, placed under the control of a control programstored in a ROM 105, checks whether an item of the read NC data is"M02", which is indicative of program end. If the item of data is "M02",numerical control processing is ended.

(3) If the item of read NC data is not "M02" indicative of program end,then the processor 102 checks whether the item of NC data is the areacutting command.

(4) If the item of NC data is not the area cutting command, theprocessor 102 executes ordinary numerical control processing.

By way of example, if an item of NC data is an M-, S- or T-functioninstruction, the processor delivers the data to a machine tool 107 viaan interface circuit 106. When the machine tool 107 generates acompletion signal indicating completion of processing for the M-, S- orT-function instruction, the processor causes the NC data reader 103 toread the next item of NC data.

If the item of NC data is path data, then the processor obtainsincremental values X_(i), Y_(i), Z_(i) along the respective axes,obtains traveling distances .sub.Δ X, .sub.Δ Y, .sub.Δ Z, which are tobe traversed along the respective axes per unit time .sub.Δ T, from theaformentioned incremental values and commanded feed velocity F, anddelivers these to a pulse distributor 108.

On the basis of the input data (.sub.Δ X, .sub.Δ Y, .sub.Δ Z), the pulsedistributor 108 performs a simultaneous three-axis pulse distributioncalculation to generate distributed pulses X_(P), Y_(P), Z_(P). Thedistributed pulses are applied as inputs to servo circuits 109X, 109Y,109Z for the respective axes, thereby rotating servomotors 110X, 110Y,110Z so that the tool is moved along the cutting path.

The processor 102, in accordance with the following formulae, updatesthe present position X_(a), Y_(a) Z_(a) along the respective axes every.sub.Δ T sec, X_(a), Y_(a), Z_(a) having been stored in a working memory112:

    X.sub.a ±.sub.Δ X→X.sub.a                  (1a)

    Y.sub.a ±.sub.Δ Y→Y.sub.a                  (1b)

    Z.sub.a ±.sub.Δ Z→Z.sub.a                  (1c)

The sign depends upon the direction of movement. Similarly, inaccordance with the following formulae, the processor 102 updatesremaining traveling distances X_(r), Y_(r), Z_(r) (the initial values ofwhich are the incremental values X_(i), Y_(i), Z_(i), respectively)every .sub.Δ T sec, X_(r), Y_(r), Z_(r) having been stored in theworking memory 112:

    X.sub.r -.sub.Δ X→X.sub.r                     (2a)

    Y.sub.r -.sub.Δ Y→Y.sub.r                     (2b)

    Z.sub.r -.sub.Δ Z→Z.sub.r                     (2c)

When the following condition is established:

    X.sub.r =Y.sub.r =Z.sub.r =0                               (5)

this means that the tool has arrived at the target position. Theprocessor 102 then causes the NC data reader 103 to read the next itemof NC data.

(5) If the item of NC data is found to be the area cutting command atthe decision step (3), the processor 102 causes the NC data reader 103to read the area cutting data and store the data in a RAM 111 until thecode indicating the end of the area cutting data is read out. It shouldbe noted that the area cutting data are as follows:

(i) data indicating surface cutting or pocket cutting (assumed here tobe the former);

(ii) data specifying the curve (closed curve) CCL of the external shapeof the are AR;

(iii) cutting path direction data (the direction of arrow A in FIG. 1,taken to be the +X direction);

(iv) shift direction data (the direction of arrow B in FIG. 1, taken tobe the -Y direction);

(v) maximum amount of cut-in D

(vi) cutting velocity;

(vii) excess thickness T; and

(viii) amount of clearance C.

(6) When the reading of the area cutting data ends, the processor 102calculates the offset curve OFC (see FIG. 1) offset from the closedcurve CCL by a distance d (=T+C) obtained by adding the excess thicknessT and amount of clearance C. It should be noted that the offset curveOFC is found through the following processing. Specifically, as shown inFIG. 4, let two straight lines specifying the closed curve CCL be S1 andS2. Straight lines S1', S2' offset from the straight lines S1, S2,respectively, by the distance d are found. The intersection P2 of thestraight lines S1', S2' is then found. The intersection P2 is on pointspecifying the offset curve OFC. Accordingly, if points of intersectionare found in a similar manner and stored in the RAM 111, the offsetcurve OFC will be obtained. Obtained together with the offset curve OFCis the offset curve OFC' offset to the outer side of the closed curveCCL by (T+C+R). Note that R is the tool radius and is stored incorrespondence with a tool number in a offset memory 113.

(7) Next, the processor 102 obtains the width W, in the shift direction(direction of arrow B), of the area bounded by the offset curve OFC.

If the coordinate values Y_(max), Y_(min) of the uppermost and lowermostpoints P_(u), P_(d), respectively, of the offset curve OFC in the shiftdirection are obtained, the shift-direction width W of the area to besurface-cut can be found from the following formula:

    Y.sub.max -Y.sub.min →W                             (4)

(8) When the width W has been found, the processor 102 determines thelength of line segments, obtained by dividing the width W into n (aninteger) equal parts, nearest a preset maximum cut-in amount D withoutexceeding the same. This length is made the actual cut-in amount P(=W/n).

(9) When the cut-in amount P has been found, the processor performs theoperation 1→i.

(10) Adopting R_(e) as the effective tool radius, the processor 102obtains the position Y of the i-th cutting path PT_(i) in the shiftdirection in accordance with the equation

    Y=Y.sub.0 +W-i·P+R.sub.e                          (5)

Note that Y₀ is the coordinate (=Y_(min)) of the lowermost point P_(d)of the offset curve OFC. The effective tool radius R_(e) is storedtogether with the tool radius R in correspondence with a tool number inthe offset memory 113. Accordingly, the effective tool radius R_(e)corresponding to a commanded tool number can be obtained by reading itfrom the memory.

The effective tool radius R_(e) is the radius of the tool that actuallyparticipates in surface cutting. FIG. 5 illustrates the relationshipbetween tool radius R and effective tool radius R_(e) in the case of aface milling machine FML. In FIG. 5, BT represents the blade edge.

(11) When the shift-direction position Y of the i-th cutting path PT_(i)has been found, the points of intersection P_(i), Q_(i) between thecutting path P_(i) and offset curve OFC' are calculated.

In a case where surface cutting is performed by unidirectional cuttingin the +X direction, whichever of the points of intersection P_(i),Q_(i) has the smaller X coordinate is the cutting starting point. Incase of back-and-forth surface cutting, the points of intersectionhaving the smaller and larger values alternate as the cutting startingpoint.

(12) When the cutting starting point and cutting end point have beenfound, the processor 102 executes path processing similar to that ofstep (4) to move the tool along the i-th cutting path PT_(i) from thecutting starting point to the cutting end point.

(13) When cutting along the i-th cutting path ends, the processor 102checks whether i=n holds.

If i=n holds, surface cutting of area AR ends and processing is executedfrom step (1) onward.

(14) If i<n holds, on the other hand, then the processor increments i inaccordance with the expression

    i+1→i

and repeats processing from step (10) onward.

In the case described above, an area cutting command is inserted into anNC tape in advance, cutting paths are generated successively by usingthe area cutting data that follow the area cutting command, and surfacecutting is performed by moving a tool along the cutting paths. However,the present invention is not limited to such an arrangement. Anarrangement can be adopted in which after the area data are inputtedfrom a keyboard, an NC tape (NC data) is created through a methodsubstantially the same as that described above and the NC tape isinputted to an NC unit to perform area cutting. However, instead ofmoving the tool in the 12th step, NC data for tool movement would beprepared.

In accordance with the present invention described above, thearrangement is such that the cutting starting point and cutting endpoint of each cutting path are situated on the offset curve offset by apredetermined amount (the sum of excess thickness, amount of clearanceand tool radius) to the outer side of the closed curve CCL specifyingthe area AR to be surface-cut. As a result, tool pass in the cuttingpath direction can be minimized in length and surface cutting can beperformed at a uniform cut-in amount (W/n every stroke) close to amaximum cut-in amount. This makes it possible to perform highlyefficient surface cutting.

We claim:
 1. A surface cutting method for cutting a surface within anarea bounded by a predetermined closed curve by moving a tool along acutting path in a predetermined direction to cut the surface within thearea, thenceforth moving the tool along an adjacent cutting pathobtained by a shift of a predetermined amount to cut the surface, andrepeating these surface cutting operations, said method characterized byincluding:a first step of obtaining an offset curve offset by apredetermined amount to the outer side of said closed curve; a secondstep of obtaining width W, in a shift direction, of the area bounded bythe offset curve; a third step of finding, from among lengths of linesegments obtained by dividing said width W into n equal parts (where nis an integer), a length closest to a predetermined maximum cut-inamount without exceeding the same, and adopting said length as an actualcut-in amount P; and a fourth step of performing surface cutting byshifting the tool by the cut-in amount P in said shift direction afterthe end of surface cutting along one cutting path PT_(i), and thereaftermoving the tool along an adjacent cutting path PT_(i+1).
 2. A surfacecutting method according to claim 1, characterized in that thepredetermined amount in said first step is T+C, where T representsexcess thickness and C represents amount of clearance.
 3. A surfacecutting method according to claim 2, characterized in that said fourthstep includes:a step of obtaining the position of the cutting pathPT_(i+1) in said shift direction; a step of obtaining points ofintersection P_(i+1), Q_(i+1) between said cutting path PT_(i+1) and anoffset curve offset by (T+C+R) to the outer side of said closed path,where T represents excess thickness, C represents amount of clearanceand R represents tool radius; a step of performing surface cutting alongsaid cutting path PT_(i+1) from the point of intersection P_(i) +1 tothe point of intersection Q_(i+1).
 4. A surface cutting method accordingto claim 3, characterized by obtaining a position Y of the i-th cuttingpath in the shift direction in accordance with the following equation:

    Y=Y.sub.0 +W-i·P+R.sub.e

where Y₀ represents the position of a lowermost end of the area in theshift direction, and R_(e) represents effective tool radius.